DCDC converters are used to convert a first DC voltage into a second, usually higher, DC voltage. This may be achieved by applying the first DC voltage to an inductance, and varying or switching the current which flows in the inductance so as to produce a series of higher-magnitude positive-going and negative-going voltage peaks which are then rectified and smoothed by an output capacitor, and output as the second DC voltage to an output load. At certain points in the switching cycle, the circuit may enter a state in which current flows in the reverse direction. Such reverse current is undesirable, because it results in increased power loss. The effects of reverse current are particularly significant in low-power DCDC converters, in which the power loss due to the reverse current may be comparable to or even greater than the power transferred from the input to the output of the converter.
In order to avoid a reverse current condition, while still maximizing the power transfer through the converter, DCDC converters have been proposed which aim to achieve optimum switching timing by dynamically tuning the control timing signals in response to a cyclic variation in voltage at the sensing node of the DCDC converter circuit. For example, the voltage may be sensed at a predetermined time at the end of each forward current (second) phase, and, if no reverse current is detected, the duration of the next forward current flow phase can be increased slightly. If a reverse current is then detected, the duration of the next forward current phase is decreased again slightly. In this way, the DCDC converter switching timing control converges to the desirable situation in which the transition from one phase to the next occurs when there is zero current flowing in the converter. In DCDC converters with a cycle frequency of tens or hundreds of kilohertz, this convergence happens rapidly.
Existing reverse current detectors typically use an analog comparator for sensing voltage. However, comparators in principle require a constant bias current to operate, which increases the current consumption of the reverse current detector. Current consumption of such a comparator is not negligible in low-power converters, and especially at higher operating frequencies of hundreds of kilohertz.
In US2009/0237039A1 it was suggested to connect the reverse current sensing node of a DCDC converter operating in discontinuous mode to the non-inverting input of a flip-flop circuit in a reverse current detector. However, this arrangement also implies a not negligible current consumption in the flip-flop circuit, not least because the new value of the sensed voltage must be clocked through the flip-flop for every cycle.